1. Field of the Invention
The invention relates to printing machines, and more particularly to an ink regulation device for a printing machine.
2. Description of the Related Art
U.S. Pat. No. 4,488,808 discloses a print inspecting device for testing printing products, in which the entire printing area is scanned by a plurality of light-emitting and light-receiving units. The units are arranged in a straight line transverse to the direction of movement of the printing material. In order to check the printing quality, the image data generated by the light emitting recovery units are compared with standard image data. In operation, light is beamed onto the printing product for each inking zone by means of a halogen or tungsten lamp and the reflected light is measured by means of two photodiodes which are sensitive in different wavelength ranges. The light intensity is measured only in two wavelength ranges, not over the wavelength continuum in the visible range. It is therefore not possible to have an absolute measurement, but only a relative measurement, as compared with a color pattern. That is, only comparative measurements with respect to a printing pattern can be carried out. In addition, there is no provision for regulating the inking zone setting means.
It is therefore an object of the present invention to improve a printing machine of the type initially mentioned, such that accurate color measurement for each inking zone is possible and that, irrespective of the subject to be printed, a correct setting of the desired color reproduction in the inking zones is produced when all the material combinations, that is to say printing ink and printing material, are known.
This and other objects are achieved according to the invention by an ink regulation apparatus that determines the ink distribution for each inking zone and each printing sheet by means of measuring elements positioned on the running printing material web or the successive printing sheets (inline ink measurement). The necessary calibration of the measuring channel is carried out by the white calibration of the paper and by full-tone and half-tone prints in the individual printing inks on sample prints by means of the printing machine. The ink coverage is set by means of hand densitometers. The sample prints are produced during the commissioning of the ink regulating system or when the requirement profile changes, that is to say when other printing inks or other printing papers are used.
Since black is obtained by color addition when the three basic colors, cyan, magenta and yellow, are printed one on top of the other, a special measuring device is necessary in order to regulate the xe2x80x9ccolorxe2x80x9d black. The measuring instrument has to find the three basic colors, cyan, magenta and yellow, as well as black again in the same ratio in the printing product as that in which the ink was broken down into these colors by the preliminary printing stage. The ratio between the basic colors must be reflected in the color spectrum, considered and, consequently, in the measured values of the product. Coordinating the filters of the preliminary printing stage and of the ink regulating instrument makes it possible for the measured values to be assigned directly, even without any test prints.
According to the invention, it becomes possible for a closed regulating circuit to be made between the preliminary stage data and the final product printed with printing inks. Changes to the operating cycle of the printing machine, such as roller spacing changes, have less influence in the regulating method according to the invention, as compared with conventional ink regulating methods, since machine changes are eliminated by precise ink regulation. Another advantage is that the printing sheet is actually assessed in zones and not only at particular points, as when color measuring strips are used. The clearly defined assignment of an inking zone and corresponding measured value makes evaluation simpler and quicker, in contrast to evaluation by cameras. The parallel and simultaneous zonal measurement of the inking on each printing sheet makes it possible to regulate more quickly than in the case of the sequential zonal sensing of all the inking zones. According to the invention, all the inking zones are recorded simultaneously. This is important, particularly for regulation in the run-up phase, in order to compensate for variations quickly.
As compared with the use of cameras, the advantages of the present invention are that there is no need to convert RGB values into the CMYK model. Problems due to spherical or chromatic aberration do not arise when the entire web width is covered.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.